Introduction to Model Checking: Advantages and Techniques

1. Model Checking Lecture 1: Specification Tom Henzinger

2. Model checking, narrowly interpreted: Decision procedures for checking if a given Kripke structure is a model for a given formula of a modal logic.

3. Why is this of interest to us? Because the dynamics of a discrete system can be captured by a Kripke structure. Because some dynamic properties of a discrete system can be stated in modal logics.  Model checking = System verification

4. Model checking, generously interpreted: Algorithms for system verification which operate on a system model (semantics) rather than a system description (syntax).

5. There are many different model-checking problems: for different (classes of) system models for different (classes of) system properties

6. A specific model-checking problem is defined by I|=S “implementation” (system model) “specification” (system property) “satisfies”, “implements”, “refines” (satisfaction relation)

7. A specific model-checking problem is defined by I|=S more detailed more abstract “implementation” (system model) “specification” (system property) “satisfies”, “implements”, “refines” (satisfaction relation)

8. Characteristics of system models which favor model checking over other verification techniques ongoing input/output behavior (not: single input, single result) concurrency (not: single control flow) control intensive (not: lots of data manipulation)

9. Examples -control logic of hardware designs -communication protocols -device drivers !

10. Paradigmatic example: mutual-exclusion protocol || loop out: x1 := 1; last := 1 req: await x2 = 0 or last = 2 in: x1 := 0 end loop. loop out: x2 := 1; last := 2 req: await x1 = 0 or last = 1 in: x2 := 0 end loop. P2 P1

11. Model-checking problem I|=S system model system property satisfaction relation

12. Model-checking problem I|=S system model system property satisfaction relation

13. Important decisions when choosing a system model -variable-based vs. event-based -interleaving vs. true concurrency -synchronous vs. asynchronous interaction -clocked vs. speed-independent progress -etc.

14. Particular combinations of choices yield CSP Petri nets I/O automata Reactive modules etc.

15. While the choice of system model is important for ease of modeling in a given situation, the only thing that is important for model checking is that the system model can be translated into some form of state-transition graph.

16. q1 a a,b b q2 q3

17. State-transition graph • Q set of states {q1,q2,q3} • A set of observations {a,b} •  Q  Q transition relation q1  q2 [ ]: Q  2A observation function [q1] = {a}

18. The translation from a system description to a state-transition graph usually involves an exponential blow-up !!! e.g., n boolean variables  2n states This is called the “state-explosion problem.”

19. State-transition graphs are not necessarily finite-state, but they don’t handle well: -recursion (need push-down models) -environment interaction (need game models) -process creation We will talk about some of these issues briefly in a later lecture.

20. Model-checking problem I|=S system model system property satisfaction relation

21. Three important decisions when choosing system properties • operational vs. declarative: automata vs. logic • may vs. must: branching vs. linear time • prohibiting bad vs. desiring good behavior: safety vs. liveness

22. Three important decisions when choosing system properties • operational vs. declarative: automata vs. logic • may vs. must: branching vs. linear time • prohibiting bad vs. desiring good behavior: safety vs. liveness The three decisions are orthogonal, and they lead to substantially different model-checking problems.

23. Three important decisions when choosing system properties • operational vs. declarative: automata vs. logic • may vs. must: branching vs. linear time • prohibiting bad vs. desiring good behavior: safety vs. liveness The three decisions are orthogonal, and they lead to substantially different model-checking problems.

24. Safety vs. liveness Safety: something “bad” will never happen Liveness: something “good” will happen (but we don’t know when)

25. Safety vs. liveness for sequential programs Safety: the program will never produce a wrong result (“partial correctness”) Liveness: the program will produce a result (“termination”)

26. Safety vs. liveness for sequential programs induction on control flow Safety: the program will never produce a wrong result (“partial correctness”) Liveness: the program will produce a result (“termination”) well-founded induction on data

27. Safety vs. liveness for state-transition graphs Safety:those properties whose violation always has a finite witness (“if something bad happens on an infinite run, then it happens already on some finite prefix”) Liveness:those properties whose violation never has a finite witness (“no matter what happens along a finite run, something good could still happen later”)

28. q1 a a,b b q2 q3 Run: q1  q3  q1  q3  q1  q2  q2  Trace:a  b  a  b  a  a,b  a,b 

29. State-transition graph S = ( Q, A, , [] ) Finite runs: finRuns(S)  Q* Infinite runs: infRuns(S)  Q Finite traces: finTraces(S)  (2A)* Infinite traces: infTraces(S)  (2A)

30. Safety: the properties that can be checked on finRuns Liveness: the properties that cannot be checked on finRuns

31. This is much easier. Safety: the properties that can be checked on finRuns Liveness: the properties that cannot be checked on finRuns (they need to be checked on infRuns)

32. Example: Mutual exclusion It cannot happen that both processes are in their critical sections simultaneously.

33. Example: Mutual exclusion It cannot happen that both processes are in their critical sections simultaneously. Safety

34. Example: Bounded overtaking Whenever process P1 wants to enter the critical section, then process P2 gets to enter at most once before process P1 gets to enter.

35. Example: Bounded overtaking Whenever process P1 wants to enter the critical section, then process P2 gets to enter at most once before process P1 gets to enter. Safety

36. Example: Starvation freedom Whenever process P1 wants to enter the critical section, provided process P2 never stays in the critical section forever, P1 gets to enter eventually.

37. Example: Starvation freedom Whenever process P1 wants to enter the critical section, provided process P2 never stays in the critical section forever, P1 gets to enter eventually. Liveness

38. q1 a a,b b q2 q3 infRuns  finRuns

39. q1 a a,b b q2 q3 infRuns  finRuns  closure

40. For state-transition graphs, all properties are safety properties !

41. Example: Starvation freedom Whenever process P1 wants to enter the critical section, provided process P2 never stays in the critical section forever, P1 gets to enter eventually. Liveness

42. q1 a a,b b q2 q3 Fairness constraint: the green transition cannot be ignored forever

43. q1 a a,b b q2 q3 Without fairness: infRuns = q1 (q3 q1)* (q2) (q1 q3) With fairness: infRuns = q1 (q3 q1)* (q2)

44. Two important types of fairness 1 Weak (Buchi) fairness: a specified set of transitions cannot be enabled forever without being taken 2 Strong (Streett) fairness: a specified set of transitions cannot be enabled infinitely often without being taken

45. q1 a a,b b q2 q3 Strong fairness

46. a q1 a,b q2 Weak fairness

47. Weak fairness is sufficient for asynchronous models (“no process waits forever if it can move”). Strong fairness is necessary for modeling synchronous interaction (rendezvous). Strong fairness is makes model checking more difficult.

48. Fair state-transition graph S = ( Q, A, , [], WF, SF) WF set of weakly fair actions SF set of strongly fair actions where each action is a subset of 

49. Fairness changes only infRuns, not finRuns.  Fairness can be ignored for checking safety properties.

50. Two remarks The vast majority of properties to be verified are safety. While nobody will ever observe the violation of a true liveness property, fairness is a useful abstraction that turns complicated safety into simple liveness.